It has long been the practice in the art to prepare acidified dairy products, as for example, cottage cheese, by inoculating milk with mixed cultures of lactic acid-forming bacteria which ferment the milk until the desired acidity is achieved. Dairy products obtained commercially by this process are known as cultured products. The word "cultured" is used generically to describe any dairy product acidified by a process involving the use of bacteria in one of the processing steps. Preparing cultured dairy products using acid-forming bacteria cultures has numerous shortcomings. Fermentation by a mixed culture of bacteria is difficult to control accurately, and there are occasions when acid development is too high or too low, resulting in a product with poor flavor and/or variation in quality and shelf life from day to day. Additionally, the fermentation process has required a long time period, e.g., up to 24 hours, from start to finish, thereby monopolizing important equipment and making the process nonamenable to continuous operation and automation techniques.
More recently, various processes for preparing cheese from chemically acidified milk without using bacterial fermentation have become known and practiced in the art. For example, U.S. Pat. No. 2,982,654 (Hammond et al) teaches a process for making cheese curd wherein an acidogen is added to milk in conjunction with a proteolytic enzyme. However, in this process, the use of acidogen alone, without first acidifying the milk, requires a long period of time to form a curd, thereby making it undesirable. U.S. Pat. No. 3,172,767 (Foster et al) teaches a process in which milk is acidified at 4.degree.-5.degree. C. and is then heated to 29.degree.-82.degree. C. whereupon a proteolytic enzyme is added to produce a sweet cheese curd. This process, however, requires additional equipment for cooling the milk and does not produce an acid cheese curd. U.S. Pat. No. 3,406,076 (Little) teaches a process in which milk must be refrigerated before addition of acidogen, thereby requiring additional equipment. U.S. Pat. No. 3,620,768 (Corbin) teaches the acidification of cold milk. British Pat. No. 1,247,415 (Battelle) also teaches the acidification of cold milk, and moreover states at page 3, lines 3 through 7, that: "It is well known that when milk is acidified at a temperature of about 70.degree. F. (21.degree. C.) to a pH of about 5.20 or below, it coagulates almost instantly into a granular water coagulation, totally unfit for making cheese." This disclosure is supported generally in "Principles of Dairy Chemistry" by Robert Jenness and Stuart Patton (John Wiley & Sons, New York, 1959) at page 310, where it is stated" . . . the sensitivity of casein to coagulation by heat is enormously increased by decreasing the pH a few tenths of a unit below the normal value for milk." In the processes of the above patents, use of low temperatures greatly increases processing costs in that longer time cycles and larger quantities of reagents, e.g., acids, proteolytic enzymes and the like, are required. Conversely, heating of acidified milk in special equipment is time consuming and unsatisfactory since the milk must be heated in a quiescent state which requires special and complex heating equipment. More recently, as set forth in U.S. Pat. No. 3,882,250 (Loter et al), a process has been developed which involves acidifying milk at a temperature of about 15.degree.-30.degree. C. and then forming cheese curd by adding acidogen and proteolytic enzyme to the warm acidified milk as it is maintained in a quiescent state.